1. Field of the Invention
The present invention relates to a method for disinfecting (microbiological reduction) drinking water or wastewater using an ultraviolet (UV) lamp that generates both UV, for irradiating the water, and ozone for entraining the ozone in the water with a vortex generating apparatus.
2. Description of Related Prior Art
The United States Environmental Protection Agency (USEPA) promulgated the final version of the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) on Jan. 5, 2006. This rule has the goal to reduce the incidence of water-borne disease associated with the protozoan parasite Cryptosporidium parvum and other pathogenic microorganisms found in surface waters (USEPA, 2006). In the LT2ESWTR, the USEPA establishes tables that require an ultraviolet (UV) dose of 58 mJ/cm2 for a 1-log (90%) virus inactivation credit, 134 mJ/cm2 for a 3-log (99.9%) credit and 186 mJ/cm2 for a 4-log (99.99%) credit. The USEPA also has provided a draft companion Ultraviolet Disinfection Guidance Manual (UVDGM) (USEPA, 2003), which gives additional recommendations on the design of UV disinfection systems and operation and validation protocols to assure that the required UV doses are achieved reliably. The UVDGM specifies that the actual validated UV dose that a reactor must deliver to achieve conformance with the UVDGM is the target UV dose in the LT2ESWTR table multiplied by a safety factor, which can vary from 1.2 and 3.0 depending on the conditions of the specific validation test, whether or not medium pressure or low pressure UV lamps are used and the specific UV reactor being used.
The UV doses for virus inactivation were derived based on the UV dose response of the adenoviruses, currently the most UV resistant set of pathogenic viruses known, even though many microorganisms, such as hepatitis A virus, rotavirus, and the enteroviruses, are significantly more sensitive to UV than are adenoviruses, as set forth in Table 1.
TABLE 1UV dose required to achieve 3-log (99.9%) inactivationof a variety of microorganisms (from Yates et al.,2006 and Chevrefils et al., 2006).UV dose (mJ/cm2)Microorganismfor 3-log inactivationAdenovirus type 2 75-119Adenovirus type 15122Adenovirus type 40155Virus inactivation credit (USEPA, 2006)143Calicivirus bovine 18Calicivirus feline21-25Coxsackievirus B320-27Coxsackievirus B520-23Hepatitis A virus12-15Echovirus16-25Poliovirus 114-38Poliovirus Type Mahoney 14Rotavirus SA-1123-27MS2 (Phaage)38-57Bacillus subtilis spores47-60Escherichia coli O157:H72.5-4.5Legionella pneumophila5-9Shigella spp. 2Salmonella spp. 3-15Cryptosporidium parvum3-9Encephalitozoon intestinalis microsporidia16-18Giardia spp.2-6
Prior to publication of the LT2ESWTR and UVDGM, a UV dose of 40 mJ/cm2 had been widely accepted in the United States as providing 4-log (99.99%) inactivation of bacteria and viruses. Table 1 indicates that UV doses below 30 mJ/cm2 are generally sufficient for 3-log (99.9%) inactivation of several waterborne viruses, including hepatitis A virus, rotavirus, and poliovirus, while a UV dose below 15 mJ/cm2 would achieve 3-log inactivation of vegetative bacteria. Table 1 also indicates that protozoan pathogens are as sensitive to UV as vegetative bacteria.
The National Sanitation Foundation (NSF) has for many years certified UV drinking water treatment units under their NSF/ANSI Standard 55, which specifies a minimum UV dose of 38 mJ/cm2. Other nations (e.g., Germany and Austria) have since 1994 required validation of UV reactors to achieve minimum UV doses of 40 mJ/cm2. Recently, the NSF amended Standard 55 (to 55A) to require a minimum UV dose of 40 mJ/cm2. Many states (and provinces in Canada) require that UV systems in use in their jurisdiction be certified by the NSF Standard 55A, thereby endorsing (by default) the 40 mJ/cm2 UV dose requirement.
Recent research (see Table 1) suggests, however, that adenoviruses exhibit considerably more UV resistance than the viruses traditionally considered being of significant waterborne concern (e.g., hepatitis A virus, rotavirus, and enteroviruses). Adenoviruses, presently listed on the USEPA's Contaminant Candidate List (CCL) (USEPA, 2005), have been found in recent research to require at least 120 mJ/cm2 and possibly as much as 200 mJ/cm2 or 4-log inactivation (Ballester and Malley, 2004; Gerba et al., 2002; Meng and Gerba, 1996; Nwachuku et al., 2005; Shin et al., 2005; Thompson et al., 2003; Thurston-Enriquez et al., 2003a) (see Table 1). Based on this research, the USEPA adopted a UV dose of 186 mJ/cm2 for a 4-log inactivation credit in the LT2ESWTR. However, assuring a delivered UV dose of 186 mJ/cm2 dose will require applied doses in the 200 to 300 mJ/cm2 range using protocols detailed in the UVDGM, depending on the safety factor required.
Most viruses (particularly the adenoviruses) are quite sensitive to chlorine or chloramines (Ballaster and Malley, 2004). Since the USEPA requires either chlorine or chloramine treatment in the distribution system for surface water drinking water treatment plants, no virus inactivation credits would normally be required if UV disinfection is used with chlorine or chloramine added to provide a residual in the distribution system. There are, however, some surface water systems that will require significant virus disinfection credit, since they currently lack the ability to maintain an adequate free chlorine contact time (CA) for viruses (e.g., systems in agricultural regions where the influent ammonia levels make the use of free chlorine cost prohibitive and systems that have uncovered finished water reservoirs and insufficient contact time between the reservoir and their closest customer's tap).
Other water systems, such as groundwater systems (not subject to the LT2ESWTR), may be significantly impacted by the high UV dose levels for virus inactivation, since many of these systems do not employ chlorine or chloramine treatment following a UV disinfection system. For such systems, no UV dose requirements are currently prescribed in National Primary Drinking Water Standards, hence, states are free to specify the UV doses for groundwater systems in their state. However, it is likely that states will apply the high UV dose values for virus inactivation from the LT2ESWTR and the UVDGM to groundwater systems. The adoption of such high UV dose limits may raise the costs of UV disinfection to the point that utilities using groundwater sources (mostly small communities) will find them uneconomical and this could affect the health of many Americans. For example, the Cadmus Group (2000) found that currently, there are more that 158,000 public ground water systems in the USA. Almost 89 million people are served by community ground water systems, and 20 million people are served by non-community ground water systems. Ninety-nine percent (99%) (157,000) of ground water systems serve fewer than 10,000 people. However, systems serving more than 10,000 people serve fifty-five percent (55%) (more than 60 million) of all people who get their drinking water from public ground water systems.
The threat of water-borne viral infection is not insignificant. Failure to properly protect against viral contamination (e.g., fecal contamination) in drinking water systems (particularly groundwater systems) could lead to disease outbreaks as shown in Table 2.
Table 1 demonstrates that UW disinfection is very effective for bacteria, protozoa and most viruses. However, it is not effective against adenoviruses. On the other hand, Thurston-Enriquez et al. (2005) found that ozone is very effective against adenoviruses. They found that at a Ct value of 0.6 mg/L min can achieve more than 4 logs (99.99%) inactivation of adenoviruses even at 5° C. For example, this could be achieved with a contact time C(t) of two minutes and an ozone residual of only 0.3 mg/L. Thus it appears that a combined ozone and UV treatment system could provide protection for all known pathogenic microorganisms.
TABLE 2Illnesses Caused by Waterborne Fecal Viral PathogensEnteric VirusIllnessPoliovirusParalysisCoxsackievirus AMeningitis, fever, respiratorydiseaseCoxsackievirus BMyocarditis, congenital heartdisease, rash, fever, meningitis,encephalitis, pleurodynia,diabetes, melitis, eye infectionsEchovirusMeningitis, encephalitis, rashfever, gastroenteritisNorwalk virus and otherGastroenteritiscalicivirusesHepatitis A virusHepatitisHepatitis B virusHepatitisSmall round structured virusesGastroenteritis(probably calciviruses)RotavirusGastroenteritisenteric adenovirusRespiratory disease, eyeinfections, gastroenteritisAstrovirusGastroenteritis
Low pressure UV lamps have two principal emissions at wavelengths of 254 nm and 185 nm, with the latter being about 8% of the intensity of the former. The 254 nm UV light is absorbed by DNA and RNA in microorganisms and leads to their inactivation (by prevention of replication). The 185 nm UV light is blocked by the quartz envelop that is used for most low pressure UV lamps; however, if one uses an envelop made of very pure quartz, it is possible to transmit the 185 nm UV light through into the air surrounding the lamp. Such UV lamps are called “ozone-producing” UV lamps, since the 185 nm UV light is absorbed by oxygen in the air leading to the production of zone. Thus a single UV lamp can produce both germicidal UV light (254 nm) and ozone (185 nm) that could be used for secondary disinfection.
Czulak and Kawchitch (1964) obtained U.S. Pat. No. 3,336,099 which describes an “ozone-producing” UV lamp. Water was treated with a UV disinfection system and ozone produced from the lamp was directed to a separate tank where the ozone dissolved and produced a secondary disinfection effect.
Coviello et al. (1979) obtained U.S. Pat. No. 4,179,616 which describes an improvement on the Czulak apparatus. Ozone produced by the UV lamp to provide secondary disinfection in a separate storage tank in which the ozone was introduced into the water.
Beitzel (1981) obtained U.S. Pat. No. 4,273,660 which describes an apparatus that collects ozone produced from the air surrounding a UV lamp and injects the ozone containing air into the water surrounding the quartz sleeve so that the UV impinges on water than contains ozone from the injection port. The apparatus is described but no results concerning the disinfection effectiveness are provided.
Venosa et al (1984) Disinfection of Secondary Effluent With Ozone/UV, J. Wat. Pollut. Control Fed. 56(2): 137-142. describes a process for treating secondary effluent in a wastewater plant. Ozonated water is subjected to UV light from 14 UV lamps. The disinfection efficiency of the ozone was increased on increasing the UV dose applied. It was determined that disinfection with UV alone was very small and set forth the conclusion that a combined UV/ozone treatment had a considerable synergistic effect.
Wang and Kurylko (1993) obtained U.S. Pat. No. 5,256,299 which describes an apparatus in which UV light impinges on water in a chamber which is also fed with ozone. The primary purpose of this apparatus is to remove volatile organic compounds from the water. No mention is made of the possible use of UV and ozone together for disinfection.
Korin (1999) obtained U.S. Pat. No. 5,935,431 which describes a treatment similar to that of Czulak and Kawchitch (1964) and Coviello et al. (1979). That is, ozone produced by a UV lamp is used to provide ozone disinfection downstream of the UV reactor.
Amirsardari et al. (2001) Effect on Ozonation And UV Irradiation With Direct Filtration On Disinfection And Disinfection By-product Precursors In Drinking Water Treatment, Environ. Technol. 22: 1015-1023. describes treatment of a raw water source for a drinking water plant first with ozone and then with UV. A reduction of the heterotrophic plate count from 960 to 680 cfu/mL in the ozone disinfection stage and down to 120 cfu/mL in the subsequent UV disinfection stage was found. There is no discussion of an attempt at any simultaneous treatment.
Collivignarelli and Sorlini (2004) AOPs With Ozone And UV Radiation In Drinking Water: Contaminants Removal And Effects On Disinfection Byproducts Formation, Wat. Sci. Technol. 49(4): 51-59. describes a study concerning the advanced oxidation treatment of raw surface water using a combined ozone/UV system. It was found that the combined system could effectively remove geosmin and 2-methylisoborneol (odiferous compounds) as well as the pesticide metolachlor. The UW doses used were very high (up to 1,400 J/cm2) and no results concerning disinfection were given.
Chang (2005) obtained U.S. Pat. No. 6,932,903 which describes an apparatus in which ozone produced by a UW lamp is injected into the water to be treated which then flows in a spiral fashion around the UW lamp and sleeve, so as to lengthen the contact time of the UW radiation with the ozone-containing water. As with the Wang and Kurylko (1993), no description of the disinfection effectiveness of this apparatus is given.
Chin and Berube (2005) Removal Of Disinfection By-product Precursors With Ozone-UW Advanced Oxidation Process. Wat Res., 39: 2136-2144. describes a combined UW/ozone system for the advanced oxidation treatment of raw surface water to examine the effectiveness of removal of disinfection by-product precursors. A considerable reduction in the levels of disinfection by-product precursors was found; however, the UV doses used (1,610 mW/cm2) were very high.